A helicopter rotor hub is the primary structural assembly for driving torque to and reacting the centrifugal loads of each rotor blade and transferring lift loads thereof to the aircraft fuselage. Common varieties of rotor hubs include articulated, hingeless and bearingless types wherein the rotor hub is characterized by the specific means for accommodating the multi-directional displacement of the rotor blades. For example, articulated rotor hubs typically employ one or more bearing elements to accommodate rotor blade excursions whereas bearingless rotor hubs utilize flexible structures, commonly termed "flexbeams", to functionally replace the bearing elements of articulated rotor hubs.
Within the class or category of articulated rotors are those which include a central hub member for driving a plurality of rotor blade assemblies via spherical multi-laminate elastomeric bearings. More specifically, the hub member includes a plurality of radial spokes and shear segments which structurally interconnect a pair of radial spokes. Each shear segment, in combination with its respective pair of spokes, form a structural loop which, depending upon the configuration of the hub member, may be vertically or horizontally oriented. Each structural loop accepts a rotor assembly yoke which is generally C-shaped and circumscribes, in looped fashion, the shear segments of the hub member. The rotor assembly yoke includes a midsection, which extends though the respective structural loop, and a pair of radial arms which are disposed on either side of the shear segment. The proximal ends of the yoke arms mount to the root end of the respective rotor blade or, alternatively, to an intermediate cuff structure. A spherical elastomeric bearing comprised of alternating layers of elastomer and nonresilient shims is interposed between the midsection of each yoke and the shear segment to accommodate the loads and motions of the respective rotor blade.
Centrifugal forces are transferred to the hub member as a compressive load in the elastomeric bearing, i.e., as the yoke bears against the innermost bearing endplate of the elastomeric bearing. The spherical configuration of the elastomeric bearing accommodates the transmission of torque to the rotor blade, provides for the transmission of lift loads to the rotor hub and, accommodates the in-plane (edgewise), out-of-plane (flapwise) and pitch change (feathering) motion of the rotor blade. U.S. Pat. Nos. 3,761,199, 4,235,570, 4,568,245, 4,797,064, and 4,930,983 illustrate articulated rotors of the type described above and are generally indicative of the current state-of-the art.
Aside from the loading requirements, the size of the rotor hub assembly, e.g., the spatial separation between the spokes of the hub member and radial arms of the yoke, is determined by the operational motion envelop of the rotor system in combination with the envelop of the elastomeric bearing. That is, the flapwise, edgewise and pitch motion of the rotor system dictates the clearance requirements, e.g., between the yoke and hub member, while the size of the elastomeric bearing influences the requisite geometry of the yoke and hub member. For example, adequate clearance must be provided between the radial arms of the yoke and the radial spokes and/or respective shear segments of the rotor hub to avoid interference therebetween as the rotor assembly yoke displaces due to blade excursions. Furthermore, the adjacent yokes and/or a yoke and an intervening rotor blade damper, must be sufficiently spaced-apart to accommodate such yoke displacement.
With respect to the elastomeric bearing, several factors determine the construction and geometry, e.g., min and max transverse diameter, cone angle and thickness, of the elastomeric laminates employed therein. Firstly, the desired fatigue life of the elastomeric bearing must be established to determine the requisite properties, e.g., durometer, shear modulus, shear allowable, etc., of the elastomeric laminates. Secondly, the centrifugally induced compressive forces are considered for determining the minimum transverse diameter and inner radius of the elastomeric laminates. That is, a minimum pressure area, transverse to the direction of centrifugal loading, is required to react the centrifugal load acting on the elastomeric laminates. Thirdly, transverse loading, such as those imposed by torque or lift loads, in combination with centrifugal loads, are considered for determining the cone angle and, consequently, the maximum transverse diameter of the elastomeric laminates. That is, a minimum support cone is required to provide buckling stability when transverse loads are applied to the elastomeric bearing or when centrifugal loads, in combination with transverse displacement, are applied thereto. Lastly, and perhaps, most importantly, the anticipated displacement due to flapping and pitch change motion must be maintained below the shear strain allowable of the elastomer layers to prevent premature failure and ensure adequate service life of the bearing assembly. Insofar as the shear displacement of each layer of elastomer is limited by its shear allowable, the total displacement must be accommodated by a plurality of laminates in series. Accordingly, the total thickness of the elastomeric bearing is a function of the flap and pitch motion requirements. These design criteria will be discussed in greater detail hereinafter.
It will be appreciated that the various criteria set forth above are interrelated and must be interatively examined to ensure that all criteria are satisfied. For example, the thickness of the elastomeric bearing, which is predominately determined by the flap and pitch motion requirements, influences the maximum transverse diameter of the bearing. That is, as the bearing thickness increases, the support cone must similarly increase to maintain the necessary transverse and flapwise stiffness. Consequently, the transverse diameter of the bearing must increase to span the distance defined by the support cone.
In meeting the above-described criteria, the transverse diameter and focal distance of the prior art elastomeric bearing present obstacles which limit the available options for the rotor hub designer. For example, as the transverse diameter increases, the distance between the radial spokes of the rotor hub and radial arms of the yoke must increase to accommodate the envelop of the elastomeric bearing. The necessity to enlarge the hub and/or yoke adversely impacts the weight and aerodynamic drag characteristics of the rotor system. Similar weight and drag penalties are incurred as the thickness of the elastomeric bearing is caused to increase due to the motion requirements. More specifically, as the thickness increases, the distance from the bearing focal point to the shear segment of the rotor hub increases. It will be appreciated that as this dimension (referred to as the "focal distance") is caused to increase, a larger clearance dimension is required between the yoke and respective shear segment to accommodate the same motion requirements. Consequently, the yoke and/or radial spokes must be enlarged, yet further, to accommodate the increased spatial requirements.
The size of the elastomeric bearing also effects the ability to orient the radial arms of the yoke in a horizontal plane. Such orientation is desirable to minimize aerodynamic drag in regions of high rotational velocity airflow (a function of the distance from the rotational axis of the rotor hub assembly) and to facilitate attachment of pitch control rods which provide pitch input to the rotor blades via the rotor assembly yoke. It will be appreciated that as the yoke assemblies are enlarged to accommodate the size of the bearing, less clearance is available between adjacent yokes or between a yoke and an intervening rotor blade damper.
A need therefore exists to provide an elastomeric bearing assembly for a helicopter rotor hub which is capable of accommodating the loads and motions thereof while minimizing the envelop of the elastomeric bearing, and particularly, the transverse diameter and focal distance thereof.